Any File-derived IOSink may throw un-handleable async exceptions into the zone, see dart-lang/sdk#43663 . Instead, just write to a file with an append mode.
Like the ProcessException thrown from zip running on a bad file, the tool should catch the ArchiveException thrown from windows implementation using package:archive.
Exit the tool after a repeated network error to download. previously we were returning null and continuing on, leading to a ProcessException when we unzipped a missing file.
